docs: Auto expand flame section on documentation home page (#1823) (#1826)

add an event listener to the docs site to listen everytime on load, then
expand the first expandable section ("flame") if the current page is
home page.

doc/_sphinx/scripts/menu-expand.js

@@ -0,0 +1,21 @@
+// Auto expand the first expandable node ("flame") when loaded.
+window.addEventListener('load', (_event) => {
+    expandFirstOnHome();
+});
+
+/**
+ * This method expands the first expandable node on the home page.
+ * 
+ * If the current page is not the home page, this method does nothing.
+ */
+// When the path name ends with index.html or an empty string, it is home page.
+function expandFirstOnHome() {
+    const parts = location.pathname.split('/');
+    const lastPart = parts[parts.length - 1];
+    const isHomePage = (lastPart == '') || (lastPart == 'index.html');
+
+    if (isHomePage) {
+        // expand the first expandable node in the toctree
+        $('li.toctree-l1').has('ul').first().addClass('current');
+    }
+}